1. Technical Field
The present invention relates to a method for driving a piezoelectric element, a piezoelectric element, and a piezoelectric element applied device.
2. Related Art
In general, the piezoelectric element has a piezoelectric layer having an electromechanical conversion characteristic and two electrodes holding the piezoelectric layer therebetween. Devices (piezoelectric element applied devices) employing such a piezoelectric element as a driving source have been actively developed in recent years. As one of the piezoelectric element applied devices, a liquid ejection head typified by an ink jet recording head is mentioned.
As materials (piezoelectric materials) of the piezoelectric layer, lead zirconate titanate (PZT; Pb(Zr, Ti)O3) having a high piezoelectric constant is mentioned, for example. On the other hand, with respect to the PZT, it has also been reported that lead (Pb) is likely to volatilize in a baking process of the piezoelectric layer (for example, refer to JP-A-2012-156160, claim 1, Paragraph [0024]). JP-A-2012-156160 indicates that oxygen deficiency occurs with the volatilization and deletion of Pb and that the oxygen deficiency present near an interface with electrodes causes a reduction in the piezoelectric characteristics.
Also when piezoelectric materials other than PZT are used, it has been reported that the oxygen deficiency may occur in the piezoelectric layer (for example, refer to JP-A-2006-86223, Paragraph [0072], FIG. 7). JP-A-2006-86223 indicates that, due to uneven accumulation of the occurring oxygen deficiency on a low-voltage side, the piezoelectric characteristics decrease.
Therefore, according to JP-A-2012-156160, nitrogen atoms are doped at oxygen sites of an ABO3 type perovskite structure of a composite oxide forming the piezoelectric layer. According to JP-A-2006-86223, when it is judged that a piezoelectric element degrades, a voltage of reverse polarity (voltage which has a high frequency and does not cause displacement of the piezoelectric layer) is applied for a certain period of time.
However, JP-A-2012-156160 requires a process of doping nitrogen. In JP-A-2006-86223, the drive of the piezoelectric element is interrupted for a predetermined time while the voltage of reverse polarity is being applied. In JP-A-2006-86223, when the drive interruption time of the piezoelectric element is reduced, the application time of the voltage of reverse polarity becomes short. In this case, the uneven accumulation of oxygen deficiency (accumulation of oxygen deficiency on low voltage side of the piezoelectric layer) cannot be suppressed, so that the life of the piezoelectric element decreases.
Thus, when the process of doping nitrogen is removed, it becomes difficult to suitably drive the piezoelectric element over a long period of time only with the former techniques. Such a problem is not limited to the case where PZT is used for the piezoelectric material and similarly arises also in the case of using materials other than PZT for the piezoelectric material. Moreover, such a problem is not limited to the piezoelectric element for use in a liquid ejection head and similarly arises also in a piezoelectric element for use in other piezoelectric element applied devices.